1. Field of the Invention
The present invention relates to an anode for a fuel cell and a fuel cell using the same.
2. Background Art
Solid polymer fuel cells, particularly solid polymer fuel cells using an aqueous methanol solution as a fuel, can be operated at a low temperature and can realize a reduction in size and weight and thus have recently been energetically studied as a power supply, for example, for mobile devices. However, the performance of the conventional fuel cell does not reach a level which can realize spread use of the fuel cell. In the fuel cell, chemical energy is converted to electric power by a catalytic reaction in an electrode, and, thus, the use of a highly active catalyst is indispensable for the development of a high-performance fuel cell.
PtRu has generally been used as an anode catalyst for fuel cells. In this case, the voltage loss caused by the PtRu catalyst is about 0.3 V with respect to a theoretical voltage of 1.21 V obtained by a catalytic reaction of the electrode. This has led to a demand for an anode catalyst having a higher activity (methanol oxidation activity) than PtRu. In order to improve the methanol oxidation activity, various methods such as the addition of other element(s) to PtRu have been traditionally studied and reported.
For example, U.S. Pat. No. 3,506,494 filed in 1966 refers to the effect attained by the addition of 10 types of metals such as tungsten, tantalum, and niobium. However, the reaction field of the catalyst reaction is on the surface of the nano-size catalyst fine particle, and the layer having a thickness of a few atoms on the catalyst surface greatly affects the catalytic activity. Accordingly, there is a possibility that, even in the case of an identical catalyst composition, the state of the catalyst surface varies depending upon the synthesis process.
For example, JP-A 2005-259557 (KOKAI) is directed to a process for producing an anode catalyst by adding group 4 to 6 metals of the periodic table to platinum (Pt) and ruthenium (Ru) by an immersion method. JP-A 2005-259557 (KOKAI) describes that the methanol activity greatly varies depending upon the order of immersion. Regarding the mixing ratio among platinum, ruthenium and the group 4 to 6 metal, this document describes only that platinum:ruthenium:additive metal weight ratio=317.7:82.3:100.
It is considered that there is a high possibility that a highly active catalyst having higher activity than PtRu can be provided by regulating the synthesis process to synthesize catalyst fine particles having a nano structure which could not have hitherto been realized. Up to now, a solution method such as an immersion method has been generally used for catalyst synthesis. The solution method, however, involves a problem that catalyst structure regulation and surface regulation are difficult for elements which are difficult to be reduced and elements which are difficult to be alloyed. The present inventor has found that the addition of a small amount of tin to PtRu can improve methanol oxidation activity (JP-A 2004-281177 (KOKAI)). However, a further improvement in methanol oxidation activity by improving the process has been desired.
The synthesis of the catalyst by sputtering or vapor deposition is advantageous from the viewpoint of material regulation. However, a few studies on the influence of the type of elements, catalyst composition, substrate material, substrate temperature and the like on the process have been made.
JP-A 2004-281177 (KOKAI) reports the effect attained by the addition of tin or tungsten to a PtRu alloy in the formation of a catalyst on a gold foil or a silicon (Si) substrate by sputtering. However, there is room for improvement in methanol oxidation activity. In JP-A 2004-281177 (KOKAI), there is no description on the effect attained by the addition of elements other than tin or tungsten. Regarding the effect attained by the addition of tin, only the effect in the case where the content of tin in the catalyst layer is 25% is reported.
As described above, studies on an improvement in activity by the addition of other elements to PtRu have been energetically made. This method, however, suffers from a problem that, under acidic conditions, the added metal is disadvantageously eluted. A perfluorosulfonic acid polymer (for example, Nafion (tradename) manufactured by Du Pont) is generally used as a proton conductive organic polymer binder in the electrode catalyst layer. Upon the contact of the perfluorosulfonic acid polymer with the catalyst, the metal added to PtRu is eluted resulting in deteriorated activity. As a result, power generation which is stable for a long period of time becomes difficult. Reducing the amount of the perfluorosulfonic acid polymer used in the catalyst layer is necessary for reducing the elution of the metal from the catalyst. The reduction in amount of the perfluorosulfonic acid polymer, however, involves a problem that the proton conductivity is lowered making it impossible to realize high output.
The present inventors have proposed in JP-A 2006-32287 (KOKAI), for example, an electrode for a fuel cell, comprising an oxide carrier such as titanium, particles of an oxide of tungsten or the like, and a binder for binding them. In the electrode catalyst layer produced by combining an oxide (an oxide superacid) of titanium, tungsten or the like, a PtRu catalyst, and a binder, the continuity between the oxide superacids and the continuity between the PtRu catalyst carrier materials are considered to be hindered by the binder. Further, the deposition of the binder on the surface of the oxide superacid and the redox catalyst is expected to cause a problem that water necessary for the generation of protons is not satisfactorily supplied to the oxide superacid or that the fuel is not satisfactorily supplied to the redox catalyst. As a result, it is considered that a three-phase interface at which an electrode reaction takes place is unsatisfactory.